This application claims priority to prior application JP 2002-20097, JP 2002-70097, JP 2002-91512, and JP 2002-93843, the disclosures of which are incorporated herein by reference.
This invention relates to a digital radio receiver for receiving an electric wave from an artificial satellite (that may be called a xe2x80x9csatellite wavexe2x80x9d) or an electric wave on the ground (that may be called a xe2x80x9cterrestrial wavexe2x80x9d) to listen in a digital radio broadcasting and, in particular, to a loop antenna for use in the digital radio receiver.
In recent years, a digital radio receiver, which receives the satellite wave or the terrestrial wave to listen in the digital radio broadcasting, has been developed and is put to practical use in the United States of America. The digital radio receiver is mounted on a mobile station such as an automobile and can receive an electric wave having a frequency of about 2.338 gigaheltz (GHz) to listen in a radio broadcasting. That is, the digital radio receiver is a radio receiver which can listen in a mobile broadcasting. In addition, the terrestrial wave is an electric wave in which a signal where the satellite wave received in an earth station is frequently shifted a little. It is noted that the satellite wave is circular polarization while the terrestrial wave is linear polarization.
In order to receive such an electric wave having the frequency of about 2.338 GHz, it is necessary to set up an antenna outside the automobile. Although such antennas have been proposed those having various structures, the antennas of cylindrical-type are generally used rather than those of planer-type (plane-type). It is possible to obtain a wider directivity by making a shape of the antenna cylindrical.
A loop antenna is known in the art as one of the antennas of the cylindrical-type. The loop antenna has structure where one antenna lead member is wound around a peripheral surface of a hollow or solid cylindrical (which is collectively called xe2x80x9ccylindricalxe2x80x9d) member in a loop fashion, namely, is an antenna having the form of a loop. The cylindrical member may be merely called a xe2x80x9cbobbinxe2x80x9d or a xe2x80x9cdielectric corexe2x80x9d in the art. In addition, the antenna lead member may be merely called a xe2x80x9clead.xe2x80x9d It is known in the art that the loop antenna acts as an antenna having a directivity in a longitudinal direction thereof if the antenna lead member has an all around length which is selected to about one wavelength. This is because the antenna lead member has a sinusoidal distribution of a current. The loop antenna is for receiving the circular polarization or the satellite wave. That is, the loop antenna is used as a satellite wave antenna.
Although it is necessary for the loop antenna to feed to it, a four-point feeding is generally adopted to the loop antenna. In order to receive circular polarization, feeding is carried out at four points having a phase difference of 90 degrees. The loop antenna with the four-point feeding is called in the art a four-point feeding loop antenna. In an existing four-point feeding loop antenna, a feeding is directly carried out to a loop portion.
More specifically, the existing four-point feeding loop antenna comprises a cylindrical body formed by rounding a flexible insulation film around a central axis in a cylindrical fashion, a loop portion made of conductor that is formed on the cylindrical body along a peripheral surface thereof around the central axis in a loop fashion, and four feeders formed on the peripheral surface of the cylindrical body to feed the loop portion at four points. The loop portion is directly connected with each of the four feeders. Such a four-point feeding loop antenna is called a directly coupling type four-point feeding loop antenna.
After the electric wave is received by the loop portion as a received wave, the received wave is divided through the four feeders into four partial received waves which are phase shifted and combined by a phase shifter so as to match phases of the four partial received waves to obtain a combined wave, and then the combined wave is amplified by a low-noise amplifier (LNA) to obtain an amplified wave which is delivered to a receiver body. A combination of the four-point feeding loop antenna, the phase shifter, and the low-noise amplifier is called an antenna device.
In the manner which is described above, inasmuch as the existing four-point feeding loop antenna directly feeds the loop portion from the four feeders, the existing four-point feeding loop antenna is disadvantageous in that it has a too high feeding impedance. Thus, the existing four-point feeding loop antenna is disadvantageous in that it is difficult to obtain an impedance match.
In addition, a monopole antenna is for receiving the linear polarization or the terrestrial wave. That is, the monopole antenna is used as a terrestrial wave antenna. A combination of the loop (or satellite wave) antenna and the monopole (or terrestrial wave) antenna is called a composite antenna. In order to receive both of the satellite wave and the terrestrial wave, an antenna unit including the composite antenna is used. The antenna unit further comprises a shield case mounting the loop antenna and the monopole antenna thereon, top and bottom covers for covering the loop antenna, the monopole antenna, and the shield case. In order to connect the antenna unit with a receiver body, a twin cable is used. The twin cable is connected to the shield case through a bushing sandwiched between the top cover and the bottom cover. The twin cable consists of a first cable for the loop antenna or the satellite wave and a second cable for the monopole antenna or the terrestrial wave. The first cable has a first connector at a tip thereof while the second cable has a second connector at a tip thereof.
On the other hands, the receiver body has a first receptacle for the satellite wave and a second receptacle for the terrestrial wave. Accordingly, the first and the second connectors must be connected to the first and the second receptacles, respectively. It is therefore necessary to distinguish between the first cable and the second cable.
It is therefore an object of the present invention to provide a four-point feeding loop antenna which is capable of easily obtaining an impedance match.
It is another object of the present invention to provide a four-point feeding loop antenna which is capable of widening an adjustment range of impedance and a frequency characteristic thereof.
It is still another object of the present invention to provide a four-point feeding loop antenna which has a high antenna gain.
It is yet another object of the present invention to provide an antenna unit comprising a twin cable which is capable of certainly distinguishing between a first cable for a satellite wave and a second cable for a terrestrial wave.
Other objects of this invention will become clear as the description proceeds.
According to a first aspect of this invention, an electromagnetic coupling type four-point feeding loop antenna comprises a tubular body formed by rounding a flexible insulator film member around a central axis in a tubular fashion. The tubular body has a peripheral surface. Made of conductor, a loop portion is formed on the tubular body along the peripheral surface around the central axis in a loop fashion. The loop portion has a loop width. Four feeders are formed on the peripheral surface of the tubular body to feed to the loop portion at four points. Each of the four feeders has a feeder width. Connected to the loop portion, four electromagnetic coupling wires extend on the flexible insulator film member from the loop portion along the four feeders with gaps left between the four feeders and the four electromagnetic coupling wires, respectively. Each of the four electromagnetic coupling wires has a coupling wire width. The loop width, the feeder width, and the coupling wire width are substantially equal to one another. Each of the gaps is laid in a range between 0.2 mm and 0.8 mm, both inclusive, when the electromagnetic coupling type four-point feeding loop antenna has a feeding impedance of a range between 25 xcexa9 and 100 xcexa9, both inclusive.
According to a second aspect of this invention, an electromagnetic coupling type four-point feeding loop antenna comprises a tubular body formed by rounding a flexible insulator film member around a central axis in a tubular fashion. The tubular body has a peripheral surface. A loop portion made of conductor is formed on the tubular body along the peripheral surface around the central axis in a loop fashion. Four feeders are formed on the peripheral surface of the tubular body to feed to the loop portion at four points. Four pairs of electromagnetic coupling wires are connected to the loop portion. Each pair of electromagnetic coupling wires extends on the flexible insulator film member from the loop portion along one of the four feeders with gaps so as to put the one of the four feeders between the pair of electromagnetic coupling wires.
According to a third aspect of this invention, a four-point feeding loop antenna comprises a tubular body formed by rounding a flexible insulator film member around a central axis in a tubular fashion. The tubular body has a peripheral surface. A loop portion made of conductor is formed on the tubular body along the peripheral surface around the central axis in a loop fashion. The loop portion has four bending portions each of which is bent towards a feeding source. Four feeders are formed on the peripheral surface of the tubular body to feed to the loop portion at four points.
According to a fourth aspect of this invention, an antenna unit comprises a satellite wave antenna for receiving a satellite wave, a terrestrial wave antenna for receiving a terrestrial wave, and a shield case mounting the satellite wave antenna and the terrestrial wave antenna thereon. Top and bottom covers are for covering the satellite wave antenna, the terrestrial wave antenna, and the shield case. A twin cable is connected to the shield case through a bushing sandwiched between the top cover and the bottom cover. The twin cable comprises a first cable for the satellite wave antenna and a second cable for the terrestrial wave antenna. The first and the second cables have first and second outer coats, respectively. At least one of the first and the second outer coats has marking formed thereon to allow to distinguish between the first cable and the second cable.